Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfer and fixing, as well as its very fast printing speed. Ink jet printing mechanisms can be categorized by technology as either drop on demand ink jet or continuous ink jet.
The first technology, “drop-on-demand” ink jet printing, provides ink droplets that impact upon a recording surface by using a pressurization actuator (thermal, piezoelectric, etc.). Many commonly practiced drop-on-demand technologies use thermal actuation to eject ink droplets from a nozzle. A heater, located at or near the nozzle, heats the ink sufficiently close to its boiling point to form a vapor bubble that creates enough internal pressure to eject an ink droplet. This form of ink jet is commonly termed “thermal ink jet (TIJ).” Other known drop-on-demand droplet ejection mechanisms include piezoelectric actuators, thermo-mechanical actuators, and electrostatic actuators.
The second technology, commonly referred to as “continuous” ink jet printing, uses a pressurized ink source that produces a continuous stream of ink droplets from a nozzle. The stream is perturbed in some fashion causing it to break up into droplets at a nominally constant distance known as the break-off length from the nozzle. Control of these droplets can be either thermally-based or electrostatically-based. In thermally-based control, pulsed currents are applied to small, ring-shaped heating elements surrounding the nozzles to heat the ink passing through the nozzle region, and form ink droplets of different sizes. A pneumatic deflector generates a current of air which deflects the trajectory of the droplets so that the smaller droplets strike a printing medium, while the larger droplets strike a recycling gutter for collection and recirculation. In electrostatically-based control, a charging electrode structure is positioned at the nominally constant break-off point so as to induce a data-dependent amount of electrical charge on the drop at the moment of break-off. The charged droplets are directed through a fixed electrostatic field region causing each droplet to deflect proportionately to its charge such that some strike a recording medium while others strike a gutter for collection and recirculation.
The print heads of continuous ink jet printers generally comprise one or more printing modules, each of which includes a manifold having a slot-like opening for supplying a pressurized flow of ink, and a die mounted over the slot-like opening of the manifold. The manifold is precision-machined from a corrosion resistant metal, such as stainless steel, to tolerances better than 1/1,000 of an inch. The manifold has an elongated, generally rectangular face that includes the slot for conducting pressurized ink. The die is an elongated, rectangular plate of silicon which overlies the rectangular face of the manifold. It is precision fabricated to form a row of many small ink jet nozzles uniformly spaced apart at close intervals to achieve high resolution printing. Below each nozzle, a high aspect-ratio cavity is etched thru the thickness of the die so that pressurized ink can pass from the manifold through the cavity and out of each nozzle. In addition to the fabricated nozzles, the die can also include integrated micro-electronic circuitry. In the case of thermally-based control, such circuitry includes a circular micro heater around each nozzle, and an electrically conductive lead connected to each micro-heater that terminates in a metal pad on the other side of the die. Microwires are provided between each of the metal pads of the die to a corresponding metal pad on a flexible interconnect, which in turn is connected to the output of a control circuit of the printer.
For printing at 600 dots per inch (dpi), the nozzle-to-neighboring nozzle separation needs to be less than 43 micrometers. To print on a standard 8.5×11 inch media, the immobile ink ejecting print head can contain a single die that is 8.5″ long. Alternatively, printing may be from two dies each about 4.3″ long, or several shorter dies. Multiple dies need to be assembled end-to-end, usually in an offset manner, to form an 8.5″ long printing engine. It is difficult to fabricate 8.5″ silicon-based print head dies due to silicon wafer size limitations. On the other hand, in order to minimize the number of end-to-end assemblies, and to maintain quality control of individual dies, the use of a multitude of short dies is not preferred. One optimum compromise is to assemble two print head modules, each of which contains a 4.3″ long die. Two such modules can then be butted together to print onto 8.5″ wide media, or multiples of such modules can be lined up for printing even wider media. For 600 dpi printing applications, about 2600 nozzles are present in a 4.3″ long die. Full-size page printing needs two such modules for each color. Consequently, for full, four color printing (using black, magenta, yellow, and cyan inks), a minimum of eight modules are needed in a continuous ink jet printer.
During assembly of the die into a print head module, it is critical that the die containing the printing jets be precisely positioned on its respective manifold so that when the manifolds of two or more modules are mounted in end-to-end relationship in the print head housing, the spacing between the last ink jet on the die of one module is spaced about 43 microns from the first jet on the die of the abutting module. If the spacing between these two ink jets of the abutting modules varies substantially from 43 microns, either a light or dark streak artifact may occur in the printed product produced by the print head, depending upon whether these two ink jets are too far or too close to one another. The tolerance for such alignment has been examined by the applicant, and it has been found that if the nozzle misalignment is less than half the nozzle to nozzle separation, i.e. less than 21 micrometers, the resulting printing quality is acceptable, especially if some printing compensation procedure is used. For example, in a nozzle misalignment situation where the first and last nozzles are closer than 43 microns, a 25-50% decrease in ejected drop volume from these nozzles can be programmed in. Conversely, if the first to last nozzle misalignment is further than 43 micrometers, then a 25-50% increase in ejected drop volume is effective in masking printing artifacts. Hence the criteria for nozzle alignment tolerances are less than one half of the nozzle to nozzle separation distance.
It is, of course, highly desirable that the print head be durable and capable of as many hours of reliable operation as possible without servicing or replacement. Continuous ink jet print heads are almost exclusively used for long runs of high volume, commercial printing where the time and costs associated with print head replacement have a substantial impact on the expenses associated with such printing. At the same time, it is also highly desirable that the module be assembled in such a way as to allow the manifold to be recycled at the end of the service life of the print head, which may be several hundreds of hours. The manifold, being precision-machined out of stainless steel, is a relatively expensive component of the print head and has a potentially long service life. By contrast, the silicon die costs less than a tenth as much as the manifold, yet has a far shorter service life. While it is important that the die be mounted on to the surface of the manifold in such a way as to achieve a precise, secure and leak-proof bond during the service life of the die, it is equally important that the die be removable from the manifold at the end of the print head service life without damage to the manifold so that it can be re-used.
Finally, it is critical that the microwiring connecting the electrodes in the die to the pads of the integrated flexible interconnect be insulated from exposure to ink and mechanical shock which could interfere with the transmission of electrical control signals to the micro-heaters surrounding the dies.
To achieve all of the aforementioned assembly objectives of precise positioning, durability, die removability, and insulation of the microwiring between the die and the integrated control circuit, the silicon dies are usually bonded over the slot-like opening of the stainless steel manifold with ultra-violet or other room temperature curable epoxy adhesives. The curing of such epoxies does not significantly change the precise positioning between the die and the manifold, and can provide a reasonably secure and leak-proof bond. Such cured epoxies further allow the die to be easily removed from the manifold without damage by the application of localized heat to the die for a relatively short time. Finally, such epoxies can be easily be applied to form a “glop top” over the microwiring during assembly of the printing module that protectively encapsulates the microwiring connecting the die contact pads to the flexible interconnect contact pads.
While the use of room-temperature or ultra-violet cured epoxies results in a durable continuous ink jet print head that fulfills all of the aforementioned criteria, the bonds created by such curable epoxy materials ultimately fail over time, largely as a result of continuous exposure to the corrosive inks used in printing. In particular, the applicant has observed that the first occurrence of bond failure is usually in the area between the glop top and the microwiring that interconnects the die with the flexible interconnect. Bond failure caused by de-lamination in the glop top area can expose the microwiring to the conductive ink, resulting in a short circuit. Alternatively, bond failure caused by swelling of the glop top can lift up the microwires above the conductive pads on the die, creating an electrical open circuit between one or more of the circular micro heaters and the flexible interconnect. Both situations will cause undesirable image artifacts. The epoxy between the die and the manifold can also be gradually corroded by the ink, eventually resulting in leakage of ink into the printer. Consequently, a longer-lived and more reliable form of die/manifold bonding and encapsulation of the microwiring is needed which maintains all of the aforementioned assembly objectives of precise die/manifold positioning and die removability.